The concepts involved in the present invention relate to the detection of PN phases of spread-spectrum signals.
Mobile communication is becoming increasingly popular. The recent revolution in digital processing has enabled a rapid migration of mobile wireless services from analog communications to digital communications.
Spread-spectrum is a method of modulation, like FM, that spreads a data signal for transmission over a bandwidth, which substantially exceeds the data transfer rate. Direct sequence spread-spectrum involves modulating a data signal onto a pseudo-random (PN) chip sequence. The chip sequence is the spreading code sequence, for spreading the data over a broad band of the spectrum. The spread-spectrum signal transmits as a radio wave over a communications media to the receiver. The receiver despreads the signal to recover the information data.
The attractive properties of these systems include resistance to multipath fading, soft handoffs between base stations and jam resistance. Additionally, in a multipath environment, the use of Rake receivers enables the harnessing of the total received energy.
Receiving the direct sequence spread-spectrum communications requires detection of one or more spreading chip-code sequences embedded in an incoming spread-spectrum signal as well as subsequent synchronization of the receiver to the detected chip-code sequence. Initial detection and phase synchronization of the spreading chip-code sequence(s) in the receiver is commonly known as code acquisition.
On the transmission side of a spread-spectrum transmitter, a pseudo random noise (PN) generator spreads the data to be transmitted. Once spread, the transmitted signal has a bandwidth that is larger than that of the data. In essence, the data is spread over a large bandwidth in the spectral domain. Once transmitted, the signal may travel over numerous paths from the transmitter to the receiver. Therefore, the receiver receives multiple signals that traveled over different paths thereby requiring the receiver to decipher the signal of each path. Furthermore, the spreading code of the multi-path signal will not be identical to the actual transmitted PN sequence due to the multi-path environment. Correlators and matched-filters are commonly used to acquire each multi-path signal.
FIG. 6 is a block diagram useful in understanding the implementation of correlators in a spread-spectrum receiver. The correlator of FIG. 6 utilizes a local PN sequence generator 602 which generates a PN sequence, also referred to as the reference code, that is the same PN sequence used as the spreading code in the transmitter. Essentially, one input of a multiplier 601 receives an input signal, as an example, a pilot signal spread by a known PN sequence. Coupled to the other input of the multiplier 601 is the local PN sequence generator 602. Therefore, the correlator first multiplies the input signal 101 with the reference code generated by the local PN sequence generator 602.
The input of an integrator 603 receives the product signal output of the multiplier 601. The product signal is integrated, over a predetermined number of N chips. N is usually equal to the PN sequence length. The integration of the signal over N chips produces a correlation value. This correlation value represents the comparison of the reference code to the sequence code of the input signal 101. High correlation values represent a close match of one multi-path signal. On the other hand, a low correlation value represents a low probability of a match.
The output of the integrator 603 goes to a decision circuit 606 comprising a threshold comparator 604 and a DSP 605. The decision circuit 606 compares the correlation value at the output of the integrator 603 to the threshold xcfx84. The threshold xcfx84 is set to a value depending on the desired probability of detecting a signal.
The DSP 605 also connects to the local PN sequence generator 602 and to each of several tracking fingers 607, where each finger 607 tracks a multipath signal. When the correlation value does not equal or exceed the threshold xcfx84, the DSP 605 sends a control signal to the local PN sequence generator 602 instructing it to advance or retard the reference code by either a half-chip or a predetermined number of chips with respect to the input signal 101. The length of the advance or retard depends on the resolution or accuracy of detection desired. Once advanced or retarded, the multiplier 601 and the integrator 603 correlate the new reference code with the input signal 101 to produce a new correlation value. This procedure repeats until the correlation value equals or exceeds the threshold xcfx84.
Each finger 607 connects to the local PN sequence generator 602 and the input signal 101 and is controlled by the DSP 605. Therefore, once the correlation value equals or exceeds the threshold xcfx84, the DSP 605 sends a control signal to the local PN sequence generator 602 instructing it to download the reference code to one of the fingers 607. The DSP 605 also sends a signal to one of the fingers 607 instructing it to receive the reference code sent by the local PN sequence generator 602. Once a finger 607 receives a reference code, it then starts to correlate the input signal 101 with the downloaded reference code signal to track the respective multipath signal.
After which, the procedure of advancing or retarding the reference code producing a new correlation value repeats as discussed above. Each time a new correlation value equals or exceeds the threshold xcfx84 represents detection of another multi-path signal. The procedure of downloading the reference code to one of the plurality of fingers 607 also repeats for each detected multi-path signal. The output of each finger is connected to the Rake combiner. The Rake combiner 608 combines each detected multipath signal, producing a combined signal 113 with low distortion and little energy loss.
This correlation technique takes a considerable amount of time. To reduce the search time, as an example, U.S. Pat. No. 5,577,022 teaches limiting the integration length to a set number of chips or an equivalent data symbol length in the forward link of an IS-95 system. With this approach, each hypothetical pilot code is correlated with the received pilot signal over a selected number of chips (e.g., 64 chips) of a PN sequence, and the results of the correlation are integrated over the same time interval to obtain a signal energy value. The result is compared to a predefined threshold. If the result is less than the threshold, the value of received signal energy associated with the hypothetical code is set to zero. If the value for one hypothetical code is set to zero, the search moves on to the next code and repeats the operations of correlating and integrating to determine the energy associated with the next hypothetical code. These operations continue until a determination is made as to the signal energy level associated with each hypothetical code in a candidate set.
As shown by this discussion, if a receiver uses only one correlator, the receiver must advance or retard and repeat the process, sequentially, to find the spreading code sequence. Repetition multiplies the delay by the number of signals that the receiver must try to find. One way to speed up this code acquisition is to use many correlators working in parallel. Some receivers use as many as 30 correlators, reducing the search time by a factor of 30. However, the amount of hardware required also increases. Potentially, it requires as many as 30 integrators and 30 comparators.
Even though simple correlators have been used in the code acquisition for reception of spread-spectrum signals, faster and more efficient techniques for code acquisition rely on matched filters. For example, U.S. Pat. No. 5,627,855 discloses a spread-spectrum matched-filter including a code generator, a programmable-matched filter, a frame-matched filter, and a controller. One object of this patented approach was to reduce cost and circuit complexity, reduce volume required, and improve the performance. Specifically, this patent provides for frame matched-filters embodied in an in-phase-frame-matched-filter and a quadrature-phase-frame matched-filter. Programmable matched filters are coupled to the outputs of the frame-matched-filters embodied in an in-phase-programmable-matched filter and a quadrature-programmable-frame-matched-filter. Timing generators, code generators, processor, controllers, and demodulators are also coupled to the filters. The patented design does reduce cost and complexity of the hardware but only with respect to conventional matched-filtering techniques. Still, the hardware required is substantial.
However, a need still exists for a technique faster than a correlator but less complex than a true matched-filter for analyzing correlation of received signals to reference codes, to identify the actual code set received. A need also exists for a hardware implementation that offers the rugged acquisition characteristic of matched-filters and the simplistic aspects of correlators.
Accordingly, a general objective of the invention is to achieve improved processing when finding a spreading code sequence in a multi-path signal.
Another object of the invention is to provide a searcher with sufficient flexibility to allow a system designer to implement the searcher in many different situations and environments.
Another object of the invention is to achieve fast, accurate spreading code detection while reducing hardware and complexity of the circuitry.
The inventive concepts alleviate the above noted problems and achieve the stated objectives by using a spread-spectrum partial matched-filter searcher system, for finding multipath signals. Specifically, the inventive searcher system utilizes a partial matched-filter for receiving an input of a spread-spectrum signal. The partial matched-filter correlates the input signal with a short segment of a PN reference sequence. The number of chips covered by the partial matched-filter at any one time is relatively small, so as to require less hardware and low circuit complexity. A decision circuit compares the correlation value with a predetermined threshold.
Another inventive concept to alleviate the above-noted problems uses the partial matched-filter, as discussed above, in combination with an integrator forming a sliding matched-filter searcher system. The partial matched-filter outputs a first correlation value, as discussed above. However, a decision circuit compares the first correlation value with a first predetermined threshold. If met, an integrator integrates the first correlation value over a predetermined number of chips. The integrator produces a second correlation value, and the decision circuit compares the value to a second predetermined threshold. If met, the searcher identified a multi-path signal.
During the integration, the PN sequence generator runs at the same speed and in the same logical direction as the input signal. After a predetermined number of chips, however, the local PN reference sequence will be frozen, and the integration circuit will be reset to zero. The inventive searcher now becomes a matched filter again. If there was success finding a multi-path signal, the coefficients of the local PN reference will be sent to a Coefficient Update Circuit. Either with success or failure of finding a multi-path signal, the DSP will check the output of the matched filter to see if the correlation value is greater than or equal to the first threshold xcfx841 after a predetermined number of chips. This process is repeated until all the significant multi-path signals are found.
This aspect of the present invention provides several advantages over the prior art. One advantage relates to the searcher functioning as a simple correlator when the partial matched-filter is reduced to a multiplier; a true matched-filter when the tap length is made at its full value; or a hybrid system of a matched-filter and a correlator. By the searcher functioning as both a matched-filter and a correlator, the searcher realizes the benefits of both the matched-filter and the correlator.
Another advantage relates to having a short tap length of the partial matched-filter having a typical value of 16 to 64, which is less than that of the prior art. With the short tap length, the circuitry of the matched-filter is not as complex and yet still providing significantly faster correlation times. Partially matching the received multi-path signal with the local PN sequence code and then successively integrating the correlation value allows the searcher to realize the benefits of a larger tap length matched-filter without the added complexity of the prior art.
Even another advantage relates to the optimization of the first and the second threshold values. The first threshold value governs whether the searcher will function as a partial matched-filter. Therefore, the first threshold value may be set to allow the matched-filter to detect a multipath signal with a certain level of probability. Once a multi-path signal has been partially detected, the integrator integrates the first correlation value until the second correlation value equals or exceeds the second threshold representing a detection of the respective multi-path signal with high probability. Accordingly, each threshold may be optimized to a desired specification, which may depend on a number of factors. This optimization enhances the flexibility of the searcher.
Another advantage relates to the tracking of each of the detected multi-path signal. Specifically, each time the second threshold is met, the local PN sequence generator downloads the reference code to one of the plurality of fingers. The finger correlates the input signal with the downloaded reference signal for fine-tuning. When the detection of all the significant multi-path signals occur, a Rake combiner receives the outputs of the plurality of fingers and combines each multi-path signal thereby limiting energy loss and signal distortion.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.